A problem to be taken account of in such heat exchangers, especially those of compact design, is that the temperature of the burnt gases is high relative to the intended final temperature of the process air to be heated and that gas-to-gas heat transfer is poor, at least slow.
Known solutions to these problems comprise the use of expensive refractory materials for the pipe-shaped combustion chamber and/or firing the burner with a large air excess in order to lower the flame temperature. The former solution has the disadvantage of a high cost price and the latter solution leads to poor thermal efficiency and high flue losses. Improving efficiency by preheating the combustion air has the disadvantage of increased NO.sub.x production.
The object of this invention is to provide a heat exchanger for heating air or another gas, in which the above-mentioned disadvantages are overcome.